1. Field of the Invention
The present invention relates to a paper feed device for a document conveyor used in an image recording device or a document image reading device for an electrophotographic copying machine and the like.
2. Description of the Prior Art
Conventionally, in an image recording device or a document image reading device, a recycling-type document conveyor (RDH) or an automatic document feeder (ADF) are used to convey documents consecutively and automatically onto a document loading tray (platen glass). In a double-sided copying device (ADU) by which a copy image is formed on both sides of a transfer sheet, a transfer sheet, on one side of which a copy has been formed, is temporarily stored in an interim stacker, after which a second paper is fed from the paper feed section and conveyed to an image forming section. With these devices the separating efficiency is extremely important.
Conventionally, a bottom feed method with superior separating efficiency has been adopted as a paper feed method. This method involves separating one document at a time from the bottom layer of a stack of paper loaded on a paper form stacker and conveying these documents consecutively to the handling section. After exposure, the document is returned to the paper form stacker or forwarded to a paper discharge stacker.
In a paper feed device which is a typical example of a recycling document conveyor (RDH) in which a bottom feed stacking system is used, a paper feed port is provided at the lower end of a paper loading tray for the paper feed device. One document sent forward from a first paper feed section adjacent to the paper feed port is passed through a first paper feed channel from a second paper feed section and introduced onto the top surface of the platen glass of a copying machine. The document is then moved to an exposure position by a conveyor belt provided on the platen glass, and in that position the exposure optical system is moved reciprocatingly to perform the exposure operation. The exposed document is once again fed by the operation of the conveyor belt, passes through a recycling channel, and is fed to the top position of a stack of documents on the paper loading tray.
In this conventional paper feed device, the first paper feed section comprises a paper feed belt which sends forward the document on the bottom of a stack of documents in the paper feed position, and a stopper roller which stops the movement of the document by applying pressure against the paper feed belt. However, with this device, when the documents aligned on the document stacker are pressed forward by a pressure belt to the paper feed position, several documents at one time move into a wedge-shaped section formed by the paper feed belt and the stopper roller and then are thrust into a nip section.
Furthermore, there are several problems produced which cause concern, such as image distortion and the like caused by soiling and chafing of the top and bottom surfaces of the document from the friction on each surface by the adjacent documents, resulting from the pressure applied by the stopper roller to the paper feed belt.
In particular, in recent years copying machines featuring high speed conveying of documents and transfer sheets, and higher paper feed stability are in demand, and a paper feed device which can convey papers of all qualities and weights is required.
A paper feed device disclosed in U.S. Pat. No. 4,345,751 comprises a means to apply a negative pressure through a plurality of through-holes provided in an endless belt for a paper feed device which feeds a document from the document loading tray to the platen glass, and an eccentric suction drum capable of a rocking motion. In this proposal, no movable document stopper is used for the document conveying device, but the documents are fed from the document loading tray to a document stop position on the opposite side of the platen glass and aligned.
With this type of multi-strand endless belt there is a tendency to produce variations in speed between the individual strands of the belt, and, as a result, distortions are produced in the paper which is being fed. If this sort of distortion is not removed prior to making the copy, an accurate copy of the image on the document is not formed on the transfer sheet. Additional drawbacks are that the construction and operation of the paper feed device is complicated. In particular, the rocking mechanism of the suction drum is complicated, and the device becomes very large.
Another conventional technology provides a paper feed device with ribs projecting from the top surface of the document loading tray, a suction box on the bottom surface, and a blower downstream from the document loading tray. Close to the center of the stack of documents on the loading tray a wave shape is formed in the document by the projecting ribs, and at the front end of this wave-shaped document an air blast from the blower is directed between the documents, and the suction from the suction box causes the documents at the bottom of the stack to be separated one at a time and fed out. (U.S. Pat. Nos. 4,284,270, 4,324,395, and 4,411,417).
However, with the paper feed device using this air knife handling method, the following problems occur.
(1) It is difficult to make the shape of the suction box conform to many different sizes of documents because the surface of the document loading tray is deformed;
(2) lead time is necessary because of the suction from the air gap. This is unsuitable for high speed paper handling;
(3) a special blower is necessary, control is complicated, and the cost is high; and
(4) because it is necessary to turn the negative pressure in the suction box on and off with a valve, extra suction time (for example, several hundred milliseconds) is inevitably produced so that there is some degree of restriction on high speed operation, and the like.
With the abovementioned bottom-feeding paper feed device, it is usually difficult to provide a stable paper feed to conform to changes in the paper size, paper quality, paper weight (thickness), and the number of sheets stacked on the paper loading tray, and the like. This is because it is difficult to control the suction pressure of the vacuum suction device with respect to the factors that produce conformity to the abovementioned various paper characteristics.
For this reason, the presently available paper feed devices are usually restricted to feeding a single document size or a document size of similar dimensions.
Also, with conventional paper feed devices, a special switching button is provided for feeding thick paper forms (corresponding to 70 to 110 kg paper), and the operator must press the button to change the setting and increase the suction pressure. However, when feeding light paper (corresponding to 40 to 45 kg paper and tracing paper) the suction pressure cannot be lowered, so that there is a tendency for the corners of the papers to be bent, or for creases to be produced in these papers.